you're fine for viewing balances, but as soon as you spend from one wallet, they'll become unsynchronised.
To elaborate, the wallet from which you spend the coins will generate a hidden "change
" address which will not be present on the other wallet*, causing you to lose a few coins (actually, you don't really lose them, they'll just only be visible (and spendable) from one wallet and not the other). However, if you always create a new copy of your wallet file after any time you spend some coins or create a new address, and remember to always transfer this up-to-date wallet file before you do anything else, there'll be no problem.
*Actually, I think the standard client generates a certain number of change addresses in advance for the sole purpose of avoiding this exact problem, but I'm not 100% sure. Best to play it safe and keep your wallet files synchronised.